1. Field of the Invention
The present invention relates to formation of a laminated body of ceramic green sheets.
2. Description of the Background Art
As a method for preparing various devices including a sensor element of a gas sensor or the like, widely known is a method (green sheet process) in which: a plurality of ceramic green sheets (hereinafter, also simply referred to as green sheets) are prepared by applying a slurry, which is a mixture of a ceramic power and an organic substance, to predetermined films and then drying; a cavity or a predetermined circuit pattern is formed by screen printing or the like in each of the ceramic green sheets as needed; the ceramic green sheets are then laminated; and a laminated body thus obtained is sintered.
In the green sheet process, a product designing and the like are performed with an allowance for a change of the size which is caused by the contraction of the green sheets (more specifically, a laminated body of them) during the sintering. Therefore, to improve the yield of the product, a size control of the green sheet is important. Particularly, due to a manufacturing method of the green sheet, the thickness of the green sheet easily varies. Therefore, an improvement of the yield based on a thickness management has been demanded. For this purpose, it is necessary to measure the thickness with a high accuracy.
A method is already known in which the thickness of a green sheet is managed by measuring the weight of the green sheet to consequently control with a high accuracy the thickness of a ceramic layer included in a sintered body which is obtained using a plurality of the green sheets (for example, see Japanese Patent Application Laid-Open No. 7-088820).
In the method disclosed in Japanese Patent Application Laid-Open No. 7-088820, it is necessary to identify a functional relationship (or a standard curve) between the weight and the thickness about every kind of the green sheet in accordance with its composition and thickness. Additionally, each time the material of the green sheet or a preparing process is changed, the functional relationship has to be revised. These are complicated.
On the other hand, a method for directly measuring the thickness of a green sheet with a certain efficiency in a sense of the process management is as follows: the green sheet is placed on a predetermined stage, and then a measurement of the thickness is performed from the upper side of the stage by means of a contact type probe using a micro gauge or the like, or a noncontact type probe using a laser beam or the like. At that time, it is also possible to measure a plurality of positions of the green sheet, by intermittently or continuously performing the measurement while moving the stage.
FIG. 6 is a partial top view showing a measuring equipment 100 which performs a measurement in such a mode. FIGS. 7A to 7E are partial side views of the measuring equipment 100 in various modes.
The measuring equipment 100 includes a stage 101 having a plate-like shape in a top view. The stage 101 has, at a lower part thereof, a bearing 104 which is threadably engaged with a ball screw 103 rotated by a stepping motor 102, and also has, at four end portions in the lower part thereof, linear guides 106 guided by guide rails 105. As shown in FIG. 7A, a green sheet 107 is placed on the stage 101, and in this condition a probe 108p of a digital micro gauge 108 is brought into contact with the green sheet 107 from the upper side of the green sheet 107, to thereby measure the thickness of the green sheet 107. By repeatedly performing the measurement while intermittently moving the stage 101, the thickness can be measured at a plurality of positions of the green sheet 107.
However, this method has a problem that since the method involves a plurality of error causes such as a mechanical accuracy of a moving mechanism which moves the stage, the flatness of the stage, and the like, these error causes are superimposed on a variation of the thickness of the green sheet itself, and consequently an obtained measurement value is not always sufficiently reliable. For example, an error is caused in the measurement of the thickness, if the guide rail 105 is mounted with an insufficient horizontal accuracy as shown in FIGS. 7B and 7C, or if there is backlash in the linear guide 106 and therefore the stage 101 does not move exactly horizontally as shown in FIGS. 7D and 7E. Specifically, since an error of 2 to 3 μm easily occurs in each of the cases, even if a variation of the thickness of the green sheet itself is equal to or less than 10 μm, this cannot be accurately measured.
On the other hand, if a plurality of green sheets constituting the laminated body have different circuit arrangements or functions depending on the position in the lamination, a required thickness accuracy is not always the same among the lamination positions. For example, in a possible case, a green sheet used for one layer is required to have a thickness error of 1% or less in the sheet, while a green sheet used for another layer is allowed to have an error of up to 5%. In such a case, if the requirement of an error of 1% or less is applied to all the green sheets, the sheet preparation yield deteriorates.